<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        var n1 = 10,
            n2 = 20;
        var n = n1++;
        console.log('n = ' + n); //  10
        console.log('n1 = ' + n1); // 11
        n = ++n1;
        console.log('n=' + n); // 12
        console.log('n1=' + n1); // 12
        n = n2--;
        console.log('n=' + n); // 20
        console.log('n2 =' + n2); //19
        n = --n2;
        console.log('n=' + n); //18
        console.log('n2 = ' + n2); //18
    </script>
</body>

</html>